One of the fundamental problems in cable design is skin-effect, a difference in electrical values encountered by a signal at different depths in a conductor, most notably resulting in power loss at high frequencies. While the problem of power loss due to skin-effect is minimal at audio frequencies, there are other problems that skin effect causes including changes in impedence and inductance throughout the cable so that different frequencies encounter different electrical values at different distances from the surface of a strand of each single conductor.
This means if a single strand is too large, skin-effect will cause different parts of a music signal to behave differently. One of the more pronounced effects is that the delicate high frequency information, the upper harmonics, become delayed in time from lower frequencies. To the ear, this means the sound is lacking in detail, is dull and is closed, not open, with a soundstage that is flat. While the energy is still there, and the frequency response has not been changed; the information content of the signal will have been changed in a way that makes it sound as though the midrange notes have lost detail, i.e., their upper harmonics.
One solution to avoid the problems caused by skin-effect is to use a single strand of copper which is just small enough to push the induced distortion caused by skin-effect out of the audio frequency range. The largest size of a strand for this purpose is about 24 awg (0.205 sq mm). However, the power transfer for a single strand of 24 awg wire is not adequate for many purposes.
There are formulas which are used to described the reduction in current and power density at greater distances from the surfaces of a conductor. However, these formulas, by themselves, do not accurately describe at what skin depth audible distortion begins. Conventional application of the skin depth formula for copper (0.0661 m.sqroot.f) yields a skin depth at 20,000 Hz of 0.467 mm which is almost one-half the diameter of an 18 gauge strand. Audible, empirical evidence shows that distortion begins at much lesser depths. The above formulas assume that a 63% reduction in current flow at the center of a conductor is acceptable, and that an 86% reduction in powder density at the center of the conductor is acceptable. Therefore, in order to provide both low resistance and low distortion, multistrand construction is necessary.
For use in certain applications, multi-strand cables of various special constructions have been developed in order to avoid power loss and phase shifts caused by skin-effect. However, the arrangements of multi-strand constructions and the materials employed have resulted in other interactions between the strands, and between the strands and the dielectrics used for support, or for other purposes. These interactions can cause phase and other distortions due to magnetic interactions between strands, inter-stand contact rectification and resistance, and energy exchanges with the supporting dielectrics.
Other problems of multi-strand construction have yet to be solved. In almost all bundles, a given strand is sometimes on the surface and sometimes on the inside of the bundle. Every time a strand leaves the surface and goes inside, some of the current (particularly the higher frequency energy), will jump to a new strand in order to stay on the surface so it may follow the path of lowest impedance. The contact between strands is less than perfect. No matter how pure the copper (for example), the surface of every strand is oxidized, and copper oxides are semiconductors. The point of contact between strands is actually a simple circuit that has capacitance, inductance, and diode rectification--contributing a host of problems. This happens thousands of times in such a cable, and is the mechanism which causes most of the hashy and gritty quality in many audio cables.
Consider the conventional litz multistrand bundle construction in which each wire is individually insulated to become what is called, "magnet wire". Part of the definition of litz is that these wires are arranged in such a geometry over a given length of cable that each strand spends an equal amount of time at the surface and on the inside of the bundle so that all strands should have the same electrical values. If some strands were always on the outside and others always on the inside, only the strands on the outside would provide the proper conducting path and all other strands would have a different property in their ability to carry the signal. By individually insulating the strands and arranging them in an equivalent geometry, as in litz, they all carry the same amount of power at a particular frequency. This conventional litz arrangement, using magnet wire (individually insulated strands), also solves the problem of distortion caused by signal crossing from one strand to another strand in order to follow the path of least resistance.
However, litz still leaves unsolved problems of magnetic interaction and dielectric energy exchange. Also, in certain applications litz presents a difficulty in having a group of insulated strands that need to be de-insulated before being attached to anything else, and there are also compromises to high quality conductive materials that result from the need to take off, not to mention to put on, the enamel or polyurethane coating that is used to individually insulate these strands. So, while litz has been an effective means for many years in dealing with some problems, it does not deal with the remainder of the problems completely and it does have its own costs, so to speak, in manufacturing and in applications.
Another problem is magnetic interaction. When a strand of copper carries a current it creates a magnetid field. When two strands carry the same current they generate two magnetic fields, which causes them to interact like two magnets. On a microscopic level, a stranded cable is actually modulated by the current going through the cable. The effect is somewhat like doppler or intermodulation distortion. The more powerful magnetic fields associated with the bass notes cause the greatest magnetic interaction, which in turn modulates the electrical characteristics of the cable which in turn modulates the higher frequencies. This is the primary reason why bi-wiring works. Speakers which use a single amplifier but have separate inputs for the bass and upper ranges are able to dramatically reduce this type of distortion. With these speakers, the cable going to the high frequency portion no longer carries the bass energy, this prevents the interaction of the magnetic fields associated with the bass notes from causing distortion to the higher frequencies.
Even if one could ensure absolute mechanical regidity in a stranded cable, the interaction between magnetic fields would still be a prime source of distortion. Much of the energy traveling through a cable is carried as magnetic fields. In most cables, the magnetic field of any given strand encounters a complex and changing series of interactions as it travels through a constantly changing magnetic environment. The magnetic field is modulated and the audio signal becomes confused and distorted.
The electrical behavior of the dielectric (insulating material) in each conductor is much more important with low level cables. Dielectric involvement, the way in which a particular material absorbs and releases energy, can have a profound effect on the musical naturalness of a signal. Dielectric constant, which is the most often quoted specification for an insulating material, is actually not very helpful in understanding the audible attributes of different materials. The coefficient of absorption gives a clearer picture but still does not tell the whole story.
The problem is that any insulating material next to a conductor acts like a capacitor which stores and later releases energy. This is true of circuit board materials, cables, resistors and of course capacitors. The ideal wire is one with no insulation except for air or a vacuum. When solid materials have to be used, they should be as electrically invisible as possible. The less energy it absorbs the better. It would be best if the energy which is absorbed stays absorbed (turned into heat), and the energy that does come back into the conductive strands should have minimal phase shift and not be frequency selective (all frequencies should experience the same behavior). The most common insulating materials are polyvinylchloride, polyethylene, polypropylene and teflon. These can be mixed with air (foamed) or applied in a way to maximize the amount of air around the metal strands. Which material is used and how it is applied will dramatically effect the performance of a low level cable but has yet to be optimized for the values of preserving the quality and original music information contained in audio signals. To date, there has not been an audio cable design which addresses and minimizes the effects of the above mentioned problems.
There is, therefore, a need for an improved low distortion cable.